A laser oscillator comprises a heat exchanger which cools a gas medium. The heat exchanger includes a cooling part which performs heat exchange between the gas medium and a cooling medium, a tubular member fixed to a frame body, and a foreign matter collection container. The tubular member is disposed so that the gas medium which flows out of the cooling part moves along an outer surface of the tubular member and then changes a proceeding direction to flow into an inlet portion. The foreign matter collection container collects foreign matters which are separated from a flow of the gas medium.